End weld control circuit

ABSTRACT

A resistance drop welding control circuit includes apparatus operable in conjunction with the weld timer of a resistance welding machine for terminating the weld interval in response to a drop of the welding resistance value to a preselected fraction of its peak value. A buffer circuit driven by the weld timer provides an output during the weld interval. A transition timer energized thereby provides a first transition signal during the initial part welding interval during which resistance irregularities and instabilities are likely to occur and a second transition signal for the remainder of the weld interval. A signal proportional to the instantaneous welding resistance is applied to an end ratio detector. The second transition signal is applied to the end ratio detector to allow it to store the peak value of the resistance signal. After the minimum time normally required to achieve a good weld has passed a minimum weld timer energizes the end ratio detector in synchronism with the alternating power supply to the welding machine to cause the end ratio detector to provide an output upon concurrence of completion of a whole number of cycles of welding current flow and a drop in the instantaneous welding resistance to a preselected fraction of its peak value. An end weld circuit energized by said output thereupon causes the weld timer to terminate the weld interval. A quality indicator responds to termination of the weld interval by the weld timer prior to energization of the end weld circuit to indicate a possible defective weld.

United States Patent Vanderhelst 1 May 9, 1972 [54] END WELD CONTROLCIRCUIT ABSTRACT [72] Inventor. Peter vanderhelst Livonia Mich Aresistance drop welding control circuit includes apparatus operable inCOnJUIlCtlOIi with the weld timer of a resistance [73] Assignee:Robotron Corporation, Detroit, Mich. welding machine for terminating theweld interval in response to a drop of the welding resistance value to apreselected frac- [22] 1970 tion of its peak value. A buffer circuitdriven by the weld timer 21 APPL ,140 provides an output during the weldinterval. A transition timer energized thereby provides a firsttransition signal during the Related US. Application Data initial partwelding interval during which resistance irregularities andinstabilities are likely to occur and a second transition [62] of July1967 signal for the remainder of the weld interval. A signalproportional to the instantaneous welding resistance is applied to anend ratio detector. The second transition signal is applied to [52] US.Cl ..219/1l0 the end ratio detector to allow it to Store the peak valueof the [Sl] Int. Cl ..B23k 9/10 resistance Signal. Af the minimum inormal, required Fleld of Search l0, 1 to achieve a good weld has passeda minimum weld timer energizes the end ratio detector in synchronismwith the alter- [56] References Cited nating power supply to the weldingmachine to cause the end ratio detector to provide an output uponconcurrence of UNITED STATES PATENTS completion of a whole number ofcycles of welding current 3,345,493 l0/l967 Guettel et al. ..219 110 anda i iflstamaneous welding resismm? preselected fraction of its peakvalue. An end weld circuit Primar Emminer j v Truhe energized by saidoutput thereupon causes the weld timer to Assismzt E'Yaminer Smithterminate the weld interval. A quality indicator responds to Am 'woodhch d & H n termination of the weld interval by the weld timer prior toney a n at W energization of the end weld circuit to indicate a possibledefective weld.

3 Claims, 4 Drawing Figures WELDING I MACHINE /0 T 1 ME R a 7/ 27 L 7WELD TZANSlTION J MlN.WEl.D 3 END r END L QUALITY SIGNAL H mgg m RATlOWELD INDCATQR BU F F E R K H- DETECTOR 7 ClRCUlT R 2. I4 /a 6; Cl RCUlTH T a EA E /67 HEAT CORRECTION CONTROL CONYROL END WELD CONTROL CIRCUITThis application is a division of my copending application Ser. No.650,789, filed July 3, 1967 now Pat. No. 3,518,395 of Aug. 30, 1970.

CROSS-REFERENCES TO RELATED APPLICATIONS:

1. Applications Ser. No. 650,714 filed July 3, 1967, 2. Application Ser.No. 650,964 filed July 3, 1967.

FIELD OF THE INVENTION DESCRIPTION OF THE PRIOR ART It has been knownfor some time that, in the welding of plain uncoated mild steel, forexample, there is a discernable change in the electrical resistance ofthe workpiece as the weld is made. For the first cycle or two of weldtime the resistance is unstable as it is affected by the conditions ofthe material surfaces and may achieve some instantaneous peaks beforesettling down. After three cycles or so the surface conditions usuallybecome stable and the resistance normally rises as the temperature ofthe workpiece rises. On the otherhand,

in some instances where the material to be welded is not properly fittedtogether, the welding resistance may fall off spuriously after the firstseveral cycles of weldingcurrent flow but before there is any effectiveweld and then as the fit-up situation changes'the weld resistance risesto its normal peak value. At the beginning of-norrnal fusion time, theresistance levels off and begins to drop gradually. It is thisresistance drop that is indicative that a weld is being made. There isevidence that the amount of drop is related to nugget size.

7 Several controls are known which do nothing more than watch resistanceunder constant current conditions and terminate weld time when thedesired resistance drop has occurred. However, these have generally beenunstable in use and relatively expensive to produce and operate. Suchcontrols have generally ignored resistance variations early in the weldinterval caused by material surface conditions and metal fit-upvariations which if uncompensated for canreadily cause false peakreadings and premature weld termination. Moreover, Applicant has foundthat the widely varying weld timers normally resulting with suchcircuitscan be limited partially by using a maximum timed interval in connectionwith the resistance drop control and further contemplates couplingsuitable heat control means thereto for controlling the characteristicof the resistance drop.

-U.S. Pat. No. 2,848,595 to Van Sciver, now assigned to the assignee ofthe present invention, discloses a circuit in which the contactresistance at the weld position is constrained to follow a predeterminedfunction during weld nugget formation. This function is experimentallydetermined prior to welding and is programmed into the welding machine.The Van Sciver machine generates a function signal related to theabove-mentioned predetermined function, compares the function signal toa signal related to the contact resistance, detects the error betweenthem and then varies the heat input to minimize the error.

However, the Van Sciver device is sharply limited in its use due to thetype of components and circuitry used therein. It has been found that adevice built according to the Van Sciver Patent requires the services ofan engineer with an oscilloscope to set it up and it was usuallynecessary to repeat the set-up at frequent intervals, often daily, dueto the tube drift and various other instabilities in the circuitry.

lt is therefore an object of this invention to provide apparatus for usewith a resistance welding machine and operable in conjunction with theweld timer of such machine for terminating the weld interval in responseto a preselected resistance drop at the weld position.

A further object is to provide an apparatus, as aforesaid, particularlyadapted and intended for use with the circuit of copending application,Ser. No. 650,714 which circuit generates a signal proportional to theinstantaneous contact resistance at the welding position during weldingand with further circuitry as disclosed in copending application, Ser.

No. 650,964 intended for constraining the heat applied to the workpieceso that the instantaneous contact resistance will follow a preselectedresistance function,.said copendingapplications both being assigned tothe assignee of the present invention.

A further object is to provide an apparatus, as aforesaid, whichincludes a highly stable and precise end ratio detector for reliablysignaling when the contact resistance has fallen to a preselectedfraction of its peak value corresponding to completion of fusion and theoutput signal of which is synchronized with the welding current waveformto insure that a whole number of cycles of welding current are appliedto the workpiece.

A further object is to provide an apparatus, as aforesaid, in which thedetermination that the weld contact resistance has reached thepredetermined fraction of its peak value is not affected by initialcontact resistance transients occurring in the initial portion of theweld interval.

A further object is to provide an apparatus, as aforesaid, in which weldcurrent will flow for the minimum time required for a satisfactory welddespite prematuredrops in resistance by said preselected fraction priorto normal fusion as a result of metal tit-up deficiencies. 4

A further object is to provide an apparatus, as aforesaid, in which theweld interval may be terminated alternatively upon said preselectedfractional drop in the weld contact resistance as a result of completionof fusion upon normal timing out of the weld timer and in the lattercase to provide an alarm to indicate a possible defective weld.

A further object of this invention is to provide an apparatus, asaforesaid, which is highly stable in operation, which is insensitive tochanges in its operating environment, which will operate for longperiods without adjustment, which can be readily and relativelyinexpensively manufactured from standard components and compactlypackaged, which is adapta' ble for use with a variety of weldingmachines and which is capable of a long and trouble-freeservice lifewith little or no maintenance.

Other objects and purposes of this invention will be apparent to personsacquainted with apparatus of this general type upon reading thefollowing description and inspecting the following drawings.

SUMMARY OF THE INVENTION In general, the objects and principles of thisinvention are met by providing a resistance drop feedback welding heatcontrol circuit of the type providing a signal related in amplitude tothe instantaneous welding contact resistance and which is capable ofconstraining the contact resistance to follow closely a preselectedresistance function, which control circuit more particularly includesapparatus operable in conjunction with the weld timer of the resistancewelding machine for terminating the weld interval before the normalfixed end point thereof in response to.a drop of the contact resistanceto a preselected fraction of its peak value corresponding to thecompletion of fusion. An end ratio device is responsive, when energized,to a drop of the contact resistance to a preselected fraction of a priorpeak value for producing an end ratio signal. An end weld circuitresponds to the end ratio signal causing the weld timer to terminate theweld interval. A buffer circuit is responsive to said weld timer forproviding an energizing signal at the beginning of the weld interval. Atransition timer is interposed between the buffer and the end ratiodetector and responds to the buffer signal for energizing end ratiodetector after a delay sufficient to allow termination of resistanceinstabilities associated with the initiation of welding.

A minimum weld timer is preferably provided and is energized by thebuffer for preventing a spurious drop in welding resistance fromtriggering the end ratio signal until time minimally sufficient forsatisfactory weld has passed. Means are provided for synchronizing theappearance of the end ratio signal with the alternating sourceenergizing the welding machine so as to provide a whole number ofwelding current cycles in the weld interval. A quality indicator ispreferably provided which provides an alarm to indicate the possibilityof a defective weld unless energized by the end weld circuit prior tonormal termination of the weld interval by the weld timer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of afeedback resistance drop circuit embodying the present invention.

FIG. 2 is a schematic diagram indicating the buffer, transition timerand minimum weld timer portions of the circuit of FIG. 1.

FIG. 3 illustrates are diagrams disclosing waveforms found in thecircuit of FIG. 2.

FIG. 4 is a schematic diagram disclosing the end ratio detector, endweld circuit and quality indicator portions of the circuit of FIG. 1.

Certain terminology will be used in the following description forconvenience and reference only and will not be limiting. The wordsupwardly," downwardly, rightwardly, and ieftwardly" will refer todirections in the drawings referred to. Such terminology will includethe words above specifically mentioned, derivatives thereof and words ofsimilar import.

DETAILED DESCRIPTION FIG. 1 discloses a feedback resistance drop weldingcontrol embodying the invention. The feedback control 10 includes an endweld control 11 more particularly embodying the present invention andwhich is cooperatively connected to the welding timer WT of theresistance welding machine not shown and to be controlled. The feedbackcontrol 10 further includes a'contact resistance generator circuit 12preferably of the type disclosed in copending application Ser. No.650,714. In addition, the control 10 includes a heat correction controlcircuit 13 preferably of the type disclosed in copending application,Ser. No. 650,964 aforementioned. The heat correction control 13 issupplied with a resistance signal R, proportional to the instantaneouscontact resistance at the welding position, through a line 14 and with asignal E proportional to the magnitude of the welding electrode voltage,preferably sampled at the amplitude peak of each welding current halfcycle and supplied from the resistance signal generator 12 through aline 16. The heat correction control 13 compares the resistance signal Rwith a preselected function which the contact resistance is to followand through a line 17 to a heat control 18 of any conventional typeassociated with the resistance welding machine, regulates said heatcontrol and thence the rate of change of contact resistance throughvariation of the welding current amplitude.

In brief, the end weld control 11 comprises a weld signal buffer circuit19 provided with a signal FIG. 38 from the weld timer WT through a line21. Such signal lasts from the beginning of the weld interval until theweld interval is terminated, either after a preselected time period,timed by any conventional means in the weld timer, usually a timingcapacitor as in U. S. Pat. No. 3,523,174, issued Aug. 4, 1970, andassigned to the assignee of the present invention, or by-the end weldcontrol 11 as hereinafter described. The weld signal buffer 19 providesan output signal (FIG. 3C) on line 21 coextensive in time with the weldtime signal from the timer WT and acts to isolate the timer WT from theend weld control 11 to avoid loading the timer output. The buffer signal(FIG. 3C) from the weld signal buffer 19 is applied through a line 22 tothe input of a transition timer 23,.

The transition timer 23 times a delay beginning with the weld interval,and hereinafter referred to as the transition time, during which thecontact resistance is likely to be unstable and subject to erraticvariations due, for example, to surface irregularities, electrodeheating and so forth. The transition timer 23 provides two outputsignals. The first, here designated the plus transition signal (FIG.3D), coincides with the transition time. The secondsignal, heredesignated the minus transition signal, appears upon cessation of theplus transition signal and continues until the weld interval isterminated. The plus and minus transition signals (FIGS. 3D and 3E,respectively,) are here positive d.c. potentials. These transitionsignals are applied through suitable respective conductors, generallyindicated by a line 24 in FIG. 1, to the heat correction control 13 toprevent storage of the peak value of the contact resistance signal Runtil after the stabilization of the welding contact resistance.

The end weld control 1 1 further includes an end ratio detector 26. TheR signal on line 14, proportional to the instantaneous weld contactresistance, is applied to the end ratio detector 26. The end ratiodetector 26 normally determines when the weld resistance has dropped toa specified fraction of its peak value. Broadly speaking, the end ratiodetector 26 functions by storing the peak value of the contactresistance signal R and then determining when the instantaneous value ofthe resistance signal R has fallen by a predetermined fraction from thepeak value stored. The minus transition signal is applied through a line27 from the transition timer 23 to the end ratio detector 26 to preventthe end ratio detector from registering a peak value of the signal Rduring the initial weld resistance instabilities normally expectedduring the transition time.

The transition timer, during the transition time, applies the positivetransition signal to a quality indicator 29 through a line 28 asdescribed hereinafter. Y

The end weld control 11 further includes a minimum weld timer 31actuated through a line 32 by the output buffer signal from the weldsignal buffer 19. The minimum weld timer provides a minimum weld signal(FIG. 3F) to the end ratio detector 26. The minimum weld signal does notprevent the end ratio detector 26 from storing the peak value of theresistance signal R but only prevents an output signal by the end ratiodetector until after a preset portion of the weld interval has passed,that portion being designated the minimum weld time and being equal tothe minimum time normally required to achieve a satisfactory weld. Theminimum weld timer prevents false end ratio detector outputs wherefaulty metal fit-up deficiencies cause a spurious large fractional dropin the resistance signal R before sufficient time has passed to allow asatisfactory weld. In addition, the minimum weld timer 31 issynchronized with the alternating power line supplying the weldingmachine so as to permit an output from the end ratio detector only atthe end of the whole number of cycles of weld current. One benefit ofsuch an arrangement is to avoid saturation of the welding transformer;

Output from the end ratio detector 26 energizes, through a line 34, anend weld circuit 36. The end weld circuit 36, on energization, causes,through a line 37, the welding timer WT to immediately terminate theweld time if it has not already timed out. before The end weld circuitalso couples to the quality indicator 29 through a line 38, the qualityindicator 29 also being energized by the buffer signal through a line 39and as aforementioned by the plus transition signal through line 28. Thequality indicator energizes a suitable alarm for wamin'g the machineoperator that the resistance signal R did not fall to the preselectedfraction of its peak value, as on completion of a satisfactory weld,weld current was shut off at the end of the normal maximum weldinginterval by timing out of the weld timer WT. On the other hand, thequality indicator 26 provides no alarm when an output from the end ratiodetector occurs prior to normal timing out of the weld timer WT,indicating that the contact resistance has fallen to the preselectedfraction of its peak resistance value and thus indicating completion ofa satisfactory weld.

Turning now to the specific circuitry of the end weld control 11,attention is first directed to FIG. 2. The buifer 19 comprises atransistor l3Q arranged in a simple amplifier or switching circuit toisolate the circuitry of the timer WT from the remainder of the end weldcontrol 11 to avoid loading said welding timer. The particular buffercircuit 19 shown is arranged to operate with a welding timer having anormal low output voltage which increases to a substantial positivevoltage,,for example 20 volts, during weld time. The buffer circuit 19includes a voltage divider comprising resistive elements 41, 42 and 43in series connection from a positive potential line 76 to a commonpotential line 63, the positive potential line .being, in the particularembodiment shown, 30 volts above the common line 63. The output of thewelding timer WT is coupled through a clamping diode 44 to a point onthe voltage divider intermediate resistors 41 and 42. The base oftransistor 310 connects between resistors 42 and 43. Thus, the lowpotential output of the timer WT at rest clamps the upper end of theresistor 42 and, hence, the base of transistor 310 at a low potentialthereby preventing conduction of transistor 310. On the other hand,during the weld interval, the weld signal rises to a high potential andthe resistor 42 is unclamped whereby the voltage divider 41, 42, 43biases the base of transistor 310 at a potential high enough forconduction.

The collector and emitter of transistor 310 are connected throughresistances 46 and 47, respectively, to the positive potential line 76and to a negative potential line 143 here held by any convenient meansat 30 volts below common line 63. The emitter of transistor 31Q isclamped at or above a voltage level somewhat below the potential of thecommon line 63 by a series string of diodes, here three in number,indicated at 48, 49 and 50 and connected between the common line and theemitter of transistor 310. Thus, during the weld intervalthe transistor3lQ conducts and its collector potential is reduced below the potentialof the common line 63 and this lowered potential appears on line 22connected to the transistor collector as the buffer signal (FIG. 3C).The buffer signal is also applied through line 32 to the minimum weldtimer 31. The line 32 includes an isolating diode 51, to allow the lowpotential buffer signal to be applied to the minimum weld timer but'toprevent the minimum weld timer from drawing current from thebuffer whentransistor 310 is nonconductive. The buffer signal is also applied toline 30 leading to the quality indicator 29.

Transition timer 23 comprises a pair of transistors 320 and 330connected as a one-shot multivibrator circuit. More particularly,transistors 32Q and 33Q are connected at their emitters directly tocommon line 63, at their collectors through respective droppingresistors 52 and 53 to the positive potential line 76 and receive basebias through respective resistances 54 and 56 from the common line 63.The base of leading transistor 32Q is coupled through a resistor 57 tothe collector of transistor 330 and the base of the latter is coupled tothe collector of the former through a series resistor 38 and capacitor59. A network comprising a series capacitor 61 and resistance 64 isparallel with a further resistance 65 connects the base of the leadingtransistor 320 to the output line 22 of the buffer circuit 19. The plustransitionsignal and the minus transition signal are taken from thecollectors of transistors 320 and 330 through lines 28 and 27respectively.

The minimum weld timer 31 includes a timing capacitor 68 connected inparallel with a load resistor 69 between the common line 63 and ajunction point 7l'which in turn is connected through an isolating diode72 and series resistance 73 to the positive potential line 76. Thebuffer signal is applied to the minimum weld timer 31 at a point betweenthe resistor 73 and diode 72 The diode 72 is oriented to allow chargingof the capacitor 68 through the resistor 73 but to prevent discharge ofthe capacitor 68 therethrough upon occurrence of a low potential on line32. I

The minimum weld timer 31 further includes a buffer circuit, comprisingcascaded emitter follower transistors 340 and 350, which applies thevoltage on the capacitor 68 to a trigger circuit comprising transistors360 and 370 while preventing such trigger circuit'from drainingthecharge from the capacitors 68. in the buffer circuit here shown, thecollectors of transistors 340 and 350 directly connect to the positivepotential line 76, base input for transistor 350 is taken directly fromthe emitter of transistor 340 and base input'for transistor 340 isobtained directly from capacitor 68 by connection to the junction point71. An emitter resistor 74 connects the emitter of transistor 350 to thecommon line 63.

The minimum weld timer 31 is also driven by a full wave rectifiedalternating supply voltage. Thus, an alternating source preferably thatfeeding the welding machine and at least in phase with the weld currentcontrolled thereby causes current flow through rectifying diodes 76 and77 on alternate half cycles of the source S. For purposes ofillustration, the source S is shown to drive a conventional transformer78 having a center tapped secondary winding 79, the center tap of whichis connected to common line 63 and the ends of which are connected tothe anodes of respective diodes 76 and 77 for alternatively causingcurrent flow therethrough. The cathodes of diodes 76 and 77 connect tothe base of triggertransistor 360 through resistors 81 and 82 while theemitter of output buffer transistor 35Q connects to said base through afurther resistor 83. Base bias is furnished to transistor 360 through abias resistor 84 from the common'line 63. Thus, the sum of the bufferedpositive voltage on capacitor 68 and the full wave rectified alternatingvoltage from diodes 76 and 77 appears on the base of transistor 360.

A capacitor 86 connects between the cathode of diode 77 and the commonline 63. The capacitor 86 charges during conduction through the diode 77sufficiently to maintain the base of transistor 360 positive, andthereby conductive, at that point in the phase of source S. Diode 77remains conductive until the diode 76 commences conduction forenergizing said transistor 360. The capacitor 86 discharges during thesucceeding half cycle of conduction (by the diode 76) through resistor82 and the conductive transistor 360. Thus, the only time in a givencycle of the supply S at which the transistor 360 is nonconductive forlack of positive bias is that point at which conduction through diode 76has just ceased and conduction through diode 77 is about to begin,capacitor68 being discharged. Connection of the rectifiers 76, 77 to thesource S is made so that initiation of conduction through diode 77corresponds to the zero degree point in the welding current waveform.Thus, the transistor 360 can be made conductive only after a wholenumber of full cyclesof weld current have occurred in the weld interval.

Trigger circuit transistors 360 and 370 are directly connected at theiremitters to the common potential line 63 and at their collectors throughdropping resistors 87 and 88 to the positive potential line 76. The baseof transistor 370 is energized from the collector of transistor 360through a resistor 79, base bias being supplied to transistor 370through a further resistor 91 from common line 63. The minimum weldsignal (FIG. 3F) is taken from the collector of trigger transistor 370through a diode 92 on line 33 and applied, as hereinafter described, tothe end ratio detector circuit 26. The minimum weld signal consists of apositive dc. potential interrupted, after the minimum weld time (i.e.,the discharge time of capacitor 68), by negative going spikes droppingto essentially the common potential when conductive bias is removed fromtransistor 360 as above described and, hence, occurring at the zerodegree points in the weld current waveform.

The end ratio detector 26(FlG. 4) includes a unijunction transistor 380,the lower base of which directly connects to the common line 63. Theemitter circuit of the unijunction 380 includes a series resistivevoltage divider comprising a resistor 93, a potentiometer 94 and afurther resistor 96 connected between the resistance signal line 14 fromthe R circuit 12 (FIG. 1) and the common line 63. Thus, a potentialproportional to the contact resistance signal R appears on the wiper ofthe potentiometer 94 and is impressed through a diode 97 on the emitterof uni unction 380. in the preferred embodiment shown, the resistancesignal R is a positive voltage and the diode 97 is thus oriented withits cathode toward the unijunction. The emitter of unijunctiontransistor 380 is also connected to the minus transition line 27 througha series diode 98 and resistor 99. The diode 98 is oriented with itsanode toward the unijunction 380 to clamp the emitter thereofessentially at common potential during the transition time before theminus transition signal, which as seen in FIG. 3B is a positive d.c.potential, appears. Thus, during the transition time, the emitter ofunijunction 38Q is clamped at a low voltage and cannot follow theresistance signal R as it rises.

A peak resistance signal storage capacitor 101 has its upper plateconnected to the emitter of unijunction 380 to be charged to whatevervoltage appears thereon. Thus, the capacitor 101 remains essentiallyuncharged during the transition time but thereafter is charged inproportion to the contact resistance signal R as it rises, the upperplate of such capacitor then carrying the rising potential on the wiperof potentiometer 94. The diode 97 allows the capacitor charge to followthe resistance signal R as it rises but prevents the capacitor fromdischarging through the potentiometer 94 when the resistance signal Rbegins to drop. Thus, the capacitor 101 stores the portion of the peakvalue of the resistance signal R appearing on the wiper of potentiometer94.

The other or lower end of the capacitor 101 connects to the base of atransistor 39Q. Transistor 39Q has its emitter directly connected to thecommon line 63 and its collector connected through a suitable droppingresistance 106 to the positive potential line 76. Transistor 39Q isnormally biased for conduction by connection of its base through aresistor 102 to the positive potential line 76 so that the lower plateof the capacitor 101 is normally shorted thereby to the common potentialline 63.

The resistance signal line and the minimum weld signal line 43 connectto the upper base of the unijunction transistor 380 through respectivediodes 103 and 104 which are oriented with their cathodes toward theunijunction transistor. The diodes 103 and 104 apply the one of theresistance signal R and minimum weld signal which is the highest inpotential at any given instance to the upper base of unijunction 38Q.Said diodes also isolate the resistance signal generator R and minimumweld timer from each other and from unwanted positive potentials fromthe end ratio detector. The outputof the end ratio detector appears as anegative spike pulse applied to the base of transistor 390 for turningsame off when the unijunction 380 fires. The unijunction fires inresponse to the combination of a charge on capacitor 101 correspondingto peaking of the resistance signal R, a drop of the resistance signal Rto or below a preselected fraction of that peak value and a negativegoing spike pulse from the minimum weld timer, as hereinafter discussedin the operation portion of the specification. v

The end weld circuit 36 comprises transistors 400 and 410 which areconnected in a one-shot multivibrator circuit. The emitters oftransistors 400 and 410 connect directly to the common line 63 and thecollectors thereof connect through respective dropping resistors 107 and108 to the positive potential line 76. Base bias is supplied to thetransistors 400 and 410 through resistors 111 and 112, respectively.Transistor 400 is normally nonconductive. The collector potential oftransistor 400 is applied to the base of transistor 410 through aresistor 113 to cause the transistor 410 to fall nonconductive whentransistor 400 starts to conduct. The collector of transistor 410connects through a series resistor 114 and capacitor 116 with the baseof transistor 400. A high collector potential on transistor 41Q chargesthe capacitor 116 at a rate fixed by the value of resistor 114, andthereby maintains the base of transistor 40Q above cut-ofi potential.Thus, the transistor 400 is conductive until the capacitor 116 charges.

The one-shot multivibrator circuit 400, 410 is energized through aresistor 117 connected between the collector of transistor 390 and thebase of transistor 400 when the end ratio occurs and causes thetransistor 390 to fall momentarily nonconductive. The end weld circuit,when energized, provides a first output to the line 37 leading to thewelding timer WT as stated above with respect to FIG. 1. The line 37 inFIG. 1 represents a pair of conductors 37a and 37b in FIG. 4. Theconductor 37a connects through an isolating diode 118 to the collectorof transistor 40Q, the diode 118 being oriented to allow conventionalcurrent flow from the weld timer through the transistor 40Q when thelatter is conductive. The conductor 37a is connected to the weld timerWT in any convenient manner, not shown, for causing same to index fromthe weld interval into the nextfunction interval for which it isprogrammed, usually the hold interval, by applying common potentialthereto on conduction of transistor 400. On the other hand, conductor37b from the weld timer connects to' the collector of transistor 400through a series network comprising a diode 119, a resistor 121 and afurther diode 122. The conductor 37b is intended to connect to thetiming capacitor, not shown, normally provided in conventional weldtimers to discharge same through the series network 119, 121 and 122when the transistor 400 becomes conductive. Although the end weldcircuit provides outputs on lines 37a and 37b particularly adapted tocontrol of weld timers of the kind disclosed in aforementioned US. Pat.No. 3,523,174, assigned to the assignee of the present invention, itwill be apparent that the present circuit is not intended to be limitedto such type of weld timer only. A further resistor 123 preferablyconnects from the positive potential line 76 to a point between diode119 and resistance 121 for limiting the extent to which conduction totransistor 400 can drop the potential on line 37b and, hence, on thetiming capacitor to limit the discharge of said capacitor to apreselected level.

The end weld circuit 29 provides a further output from the collector oftransistor 410 through a resistor 124 to the base of ttansistor 420 inthe quality indicator 29. The quality indicator circuit 29 includes aflip flop circuit comprising transistors 420 and 430. Transistors 420and 430 have emitters directly connected to the common line 63 andcollectors connected through dropping resistors 126 and 127,respectively, to the positive potential line 76. Base bias is applied totransistors42Q and 430 through suitable resistors 128 and 129,respectively. The transistors 42Q and 430 are connected base tocollector with each other through corresponding resistors 131 and 132 tomaintain same in opposite states of conduction in a conventional manner.The line 28 from the transition timer 23 upon which the plus transitionsignal appears during the transition time is impressed through aresistor 133 onto the base of transistor 430 to render it conductiveuntil turned off by conduction of the transistor 420 as a result ofoccurrence of the end ratio or, should that not occur before the end ofthe weld interval by a subsequent resetting of the flip-flop ashereinafter described.

A resistive voltage divider comprising series resistors 137, 138 and 139extends between the positive potential line 76 and the common line 63. Atransistor 44Q connects at its base between the divider resistors 138and 139, the divider 137, 138, 139 normally biasing same for conduction.A diode 134 connects the collector of transistor 420 to a junction point136 near the positive potential end of said voltage divider. The outputline 39 from the bufler circuit 19 connects through a diode 141 to thesame junction point 136. The diodes 134 and 141 define an OR gatewhereby a low potential on line 39, i.e., the buffer signal, or on thecollector of transistor 420 will clamp the junction point 136 andtherefore the base of transistor 440 at a low potential to preventconduction of said transistor.

The emitter of transistor 44Q connects to the common line 63 and itscollector connects to a positive potential source through a parallelload network for energizing same upon conduction of transistor 44Q. Saidnetwork preferably comprises a suitable no weld alarm 142 such as a lampor other indicating device and any other desired means, for example,suitable relays 144 for resetting portions of the welding machine,adjusting workflow therepast or the like.

A reset circuit ,146 is provided for turning of the transistor 440 andits alarm network by resetting the flip-flop circuit 420, 430 of thequality indicator. The flip-flop 420, 430 is reset by applying apositive potential to the base of transistor 420 to render sameconductive; this conduction in turn clamps the base of transistor 440 tode-energize the alarm network 142, 144. The reset circuitry in theparticular embodiment shown includes a manual reset portion including amanual reset switch 147 disposed between the positive supply 76 and thereset line 148 and manually closable to impress a positive potential onthe transistor 42Q to reset the quality indicator as above described. Anisolating diode 149 is placed between the manual reset portion and thepositive supply line 76. Further, filter networks 151 and 152 are placedon either side of the manual reset switch 147 to prevent switchingtransients from being placed upon the positive supply line 76 or thereset line 148. Each network 151 and 152 here comprises a pair ofresistors 153 and 154 in series with the reset switch 147 and a filtercapacitor 156 connected between the resistors 153 and 154 and the commonline 63.

The reset circuit, in the particular embodiment shown, further includesan automatic portion comprising a series capacitor 157 and resistor 158connected between the negative voltage supply 143 and a series resistor159 and isolating diode 161 link to the reset line 148. A normallyclosed switch contact 162 is connected across the capacitor 157 andresistor 158 to maintain the capacitor 157 discharged. A chargingcircuit for the capacitor 157 comprises a resistor 163 and potentiometer164 connected in series with a manual switch 166, which is closed whenit is desired for the automatic portion of the reset circuit 146 to beoperable, and a further switch contact 167 series between the positivepotential line 76 and the upper end of resistance 158. The switchcontact 167 is normally open and the contacts 162 and 167 are preferablyswitched from their normal states upon energization of one of the relays144. Upon such reversal, the contact 162 no longer shorts the capacitor157 and contact 167 supplies charging potential to the capacitor 157from the positive supply line 76 at a rate determined by the setting ofreset potentiometer 164. In addition, a line 168, including a dc.blocking capacitor 169, connects between the resistors 158 and 159 toapply a series of positive pulses thereto from any convenient source. Inthe particular embodiment shown, the pulse train is preferably theswamping pulse train appearing on the collector of transistor 6Q of theaforementioned copending application Ser. No. 650,714 at the beginningof each cycle of weld current. As a result, once the capacitor 157 hascharged sufficiently through the reset potentiometer 164, the positivepulses transmitted through capacitor 169 are elevated sufficiently inpotential to pass diode 161 and appear on thebase of transistor 42Q.Transistor 420 then fires, shutting off transistor 430 and the alarmmeans 142 and relay means 144. The quality indicator is thus reset for afurther weld interval.

OPERATION Although the broad operation of the present circuit has beenindicated generally above with respect to the description of FIG. 1 anddetails thereof have been given in the description of the specificcircuitry of FIGS. 2 and 4, the operation of the circuit will now bereviewed to insure a complete understanding of the invention.

Considering FIG. 2 then, the signal impressed by the weld timer WT onthe diode is normally a very low dc. voltage which switches to apositive voltage (for example, about volts) at the beginning of the weldinterval and remains so during the weld interval. This weld time signalis allowed to drive the base of transistor 310 so that the collectorvoltage thereof will normally be high and will switch to a slightlynegative value during the weld time. This slightly negative value isreferred to as the weld buffer signal (FIG. 3C).

As has been mentioned, the weld resistance cannot be depended upon forthe first one or two cycles of weld time as surface irregularities,electrode seating, etc., are having effect and unpredictable signalvariations may occur. For this reason, the transition timer circuit 23is used to "blank out" the first few cycles of weld time fromconsideration by the control system 10.

Prior to the weld interval, the bases of both transistors 320 and 330 ofthe one-shot" multivibrator circuit are driven by the bufier signal sothat both transistors conduct. When weld interval starts, the dc. basedrive is removed by conduction of transistor 310 and transistor 320 iscut off by capacitive coupling from the collector circuit of transistor31Q so that transistor 330 will continue to conduct for a perioddefining the transition time. Some time later (e.g., approximately threecycles), determined by circuit constants, the circuit will switch to astable state with transistor 320 conducting and transistor 330 cut off,thereby ending transition time. The net result is that the collectorvoltage of transistor 320 is low at rest, high for transition time, andlow for the remainder of weld interval and provides the positivetransition signal. In addition, the collector voltage of transistor 330is low at rest, low for transition time, and high for the remainder ofweld time and provides the negative transition signal.

It is sometimes possible, during a weld, for the weld resistance signalto behave erratically for the first few cycles of weld time. Thiscondition is normally compensated for by the transition timer. Itoccasionally occurs, however, that a weld condition might be such as tocause premature drop in the resistance signal R (mostly due to metalfit-up problems) which cannot be corrected by the heat correctioncontrol 13 (FIG. 1). It has been found experimentally that suchconditions almost always correct themselves early in the weld time. Ithas further been determined experimentally that the system of weldcontrol provides maximum advantage over conventional weld control whenthe weld interval is other than very short. It has been concluded thatthere is an advantage in preventing the time termination of a weld at aweld time below some minimum value to allow metal fit-up problems tocorrect themselves since this required minimum time period of weldingwill probably be less than the minimum practical controlled weld time.For these reasons, the minimum weld timer 31 has been incorporated intothe circuitry 11. This minimum weld timer does not prevent the storageof thepeak value of weld resistance R in the end ratio detector 26; itmerely prevents thegeneration of an output signal prior to the minimumweld time. This circuit also serves another purpose. It is desirablethat lead-trail" firing of the ignitron contactor, not shown, of thewelding machine (as shown, for example, in the afore-mentionedapplication, 'Ser. No. 650,714 be preserved to insure a whole number ofcycles of weld current through the welding transformer to preventsaturation of the transformer iron. The synchronizing circuitryincluding diodes 76 and 77 is combined with the minimum weld timer 31 toachieve this end.

The timing capacitor 68, unclamped by the weld buffer circuit 19 at rest(i.e., other than during the weld interval), is allowed to charge. Thepotential at the lower end of resistor 73 is clamped low by weld buffertransistor 31Q conduction during weld time so that the timing capacitor68 is allowed to discharge through its load resistor 69. The voltageacross this capacitor is buffered out by transistors 34Q and 350 todrive transistor 360.

Because transistors 360 and 370 are connected in a trigger circuit, thedrive (derived from the charge on the timing capacitor 68) to transistor360 causes transistor 370 to cut off so that its collector circuit will,in turn, provide a voltage,

the minimum weld signal, to the end ratio detector 26 which prevents anoutput from the end ratio detector. Eventually, the charge on timingcapacitor 68 will bleed off to the point where the end ratio detectorwill be allowed to operate.

The transistor 36Q'of minimum weld timer is also driven by the full waverectified a.c. voltage from the supply S so that transistor 370 canconduct only at the 0 and points on the voltage wave. Also, there iscapacitive filtering applied by the capacitor 86 to the first half cycleof rectified supply voltage so that transistor 37Q cannot conduct at the180 point of the supply waveform. The effect of this circuitry is that37Q can conduct only at the point on the line voltage wave (the end ofany particular line voltage cycle). It will be recalled that the endratio detector 36 can function only when transistor 370 conducts; thusthe end ratio detector must function at the end of a line voltage cycleor not at all. In this manner, the termination of weld time is achieved"synchronouslyd with the line and with the weld current which the linesupplies.

It is the function of the end ratio detector to detect the occurrence ofthe end ratio. The end ratio is defined as that state of affairs whereinthe ratio of the weld resistance to the peak weld resistance hasdecreased to some particular ratio. For example, at the highest value ofweld resistance, early in the weld time, the ratio is 1. Later in theweld time, when the weld resistance has decreased 20 percent, the ratiois 0.8. The end ratio is that ratio at which a good weld has been made.This is normally in the 0.75 to 0.85 range.

The weld resistance signal R is applied to the percent changepotentiometer 94 and a portion of the resistance signal R is appliedthrough the diode 97 to the storage capacitor 101 which stores thisvalue and applies it to the emitter of the unijunction transistor 38Q.Due to the action of the diode 97, the emitter signal will beproportional to the highest value of contact resistance reached duringthe weld period. (The storage capacitor is reset to a condition ofdischarge through 33Q during transition time at the beginning of eachweld).

The instantaneous weld resistance signal R is also applied to the base 2electrode (i.e., the upper base) of the unijunction transistor 380during each negative spike of the minimum weld signal.

With the stored portion of the highest value of weld resistance signalimpressed on its emitter and the (eventually) decreasing value of weldresistance signal R impressed on its base 2, the unijunction transistor38Q will conduct when the ratio of actual weld resistance to peak weldresistance decreases to a certain value.

The condition for firing a unijunction transistor is:

Ne nVb2 where Ve is the voltage impressed between the emitter and base1, n is the intrinsic stand-off ratio of the transistor (3 constant),and Vb2 is the voltage impressed between base 1 and base 2. Thisequation may be rewritten:

nVb2 Ve Due to circuit considerations,

Ve Kl Rp Where K1 is the potentiometer setting and Rp is the highestvalue reached by the weld resistance signal, and:

Vb2 R, Therefore:

nR K 1 Rp and if,

K2 K l/n, then:

R K2 Rp and R/Rp K2 showing that the unijunction 380 will conduct whenthe ratio of instantaneous weld resistance to highest weld resistancehas decreased to a value set by the potentiometer 94.

Conduction of unijunction 380 discharges the storage capacitor andcouples a negative pulse to the base of transistor 390 causing same tocut ofi.

During the weld interval, transistor 41Q of the one-shot circuit 400,4lQ is normally conductive. As stated, operation of the end ratiodetector causes transistor 390 to cutoff. The resulting high collectorpotential on transistor 390 turns on transistor 400 and cuts offtransistor 410 for a short pulse period determined by the time constantof the circuit. Conduction of transistor 40Q places a momentary shortcircuit from conductor 37a to common line 63 which causes the weld timerWT to index from weld time into hold time Also, conductor 37b ismomentarily short circuited to common to discharge the customary timingcapacitor, not shown, in the timer WT to prepare it for the charging forhold timing. This short circuit condition on conductor 37b lasts foronly a few milliseconds so that there is no interference with the holdtiming function in the weld timer WT.

The above discussion has assumed the end ratio to be reached, indicatingcompletion of a good weld, before the weld timer completes its timing ofthe preset weld interval, here the maximum time allotted for a weld. 0nthe other hand, it is desirable that an indication be provided when thedesired weld resistance drop does not occur during the allotted weldinterval. Practically, the weld interval cannot be allowed to continuewithout limit so some maximum limit must be set by the weld timer. Thus,should the welding machine be unable to produce a good weld within thismaximum time, the machine operator should be so informed that correctiveaction can be taken. For this reason, the quality indicator circuit 29has been included to indicate that a weld has not been completed in theweld interval.

Transistor 42Q of the flip-flop circuit of the quality indicator 29 isnormally conductive. When weld interval starts, the positive transitiontime signal drives transistor 430 conductive causing transistor 42Q toturn off. Thereafter the positive going end ratio signal from thecollector of transistor 41Q will turn on transistor 42Q to cutofftransistor 43Q if a good weld is made. If a good weld is not made, noend weld signal appears and transistor 430 will remain conductive afterthe weld timer WT ends the weld interval.

Transistor 440 is normally not conductive at rest because its base driveis clamped by conduction of 42Q. Although transistor 42Q cuts off at thestart of weld time, buffer transistor 31Q conducts to clamp the basedrive to transistor 44Q for maintaining same nonconductive. If a goodweld is made, 420 will thereafter conduct and thereby maintaintransistor 440 off after the weld interval ends. On the other hand, if agood weld is not made, cessation of conduction (at the end of weld time)of bufi'er transistor 310 will allow transistor 440 to conduct.Conduction of transistor 44Q which indicates that a bad or questionableweld has been made, energizes suitable relays 144 to operate externaland auxiliary circuits and the no-weld" warning light 142 to signal theoperator that the control system 10 is not able to guarantee the weld.

When transistor 430 remains conducting after weld time, to signify aquestionable weld, the indicating system 29 may be reset by causingtransistor 42Q to conduct. This may be done by operating the reset pushbutton 147 to provide momentary base drive to transistor 420 or resetmay be accomplished automatically. More particularly, operating of theindicating circuit 29 energizes relay 144 which by reversing the stateof contacts 162 and 167 unclamps the charging circuits 166, 163, 164,158 for the capacitor 157. As this capacitor is charged, the voltageacross it eventually increases to reduce the reverse bins on the diode149 in the drive line 148 for transistor 42Q. Capacitively coupledpositive pulses from any convenient source, preferably from the swampingpulse circuit of copending application Ser. No. 650,714 from thecollector circuit of 60 are applied to the diode 161 so that when thecapacitor voltage rises enough, the pulse voltage will pass the diode161 to provide drive for transistor 42Q to reset the circuit.

Thus, the circuitry 11 described supplied with a signal R representingthe instantaneous weld contact resistance provides a feed back controlsystem to control weld time as a function of weld resistance drop.However, the circuitry 11 further is particularly adapted for use withthe heat correction control 13 to control the weld current in such amanner that the weld time correction required to be provided by thepresent circuitry 11 is minimized.

Although a particular preferred embodiment of the invention has beendescribed above for purposes of illustration, it will be apparentthatmodifications and variations within the scope of the appended claims arefully contemplated.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

I. In a resistance welding machine including a generator for providing aresistance signal proportional to the instantaneous contact resistanceat the welding position and a weld timer for timing a preselected weldinterval and signaling the beginning and end thereof, apparatus forcausing the weld timer to terminate the weld interval before its normalconclusion in response to a drop of the weld resistance to a preselectedfraction of its peak value during the weld interval, indicatingcompletion of the satisfactory weld, such apparatus comprising thecombination:

a source of energizing potential supplied at positive and negativepotentials related to a common potential;

a buffer transistor disposed between said negative and positivepotential lines for providing a negative buffer signal upon initiationof weldtime by said weld timer;

a one-shot multivibrator circuit responsive to said negative buffersignal for providing a first transition signal during the initialportion of the weld interval and a second transition signal initiatedconcurrently with discontinuance of said first transition signal andterminating at the end of the weld interval, said first and secondtransition signals appearing on different output terminals of said oneshot multivibrator;

a timing capacitor, means for charging said capacitor from said positivepotential line and clamping means connecting said charging means to theoutput of said buffer transistor for allowing said capacitor to chargeprior to the weld interval and to clamp said charging circuitinoperative during the weld interval;

a load resistor across said timing capacitor for discharging same at apredetermined rate and buffer means providing the instantaneouspotential on said capacitor as an output thereof;

a full wave rectifier energized by an alternating current sourcesynchronized with the weld current in said welding machine and filtermeans for allowing the potential across the full wave rectifier to dropessentially to zero at the zero degree point of the welding currentwaveform but preventing such a reduction at the l 80 point;

a trigger circuit and means adding the output of said full waverectifier and said buffer means at the input to said trigger circuit,said trigger providing a minimum weld signal comprising a high potentialmaintained until said timing capacitor discharges and thereafterinterrupted by a negative going pulse at each 0 point of the weldingcurrent waveform, the load resistor of said capacitor being ofsufficient size to cause said capacitor to time out upon completion ofthe minimum time period after initiation of the weld interval requiredto achieve a satisfactory weld;

a unijunction transistor having a high potential side, a memorycapacitor coupled to the emitter thereof and means for supplying asignal proportional to the contact resistance at the welding head;

means for applying a portion of said resistance signal to said memorycapacitor for charging same, means normally clamping said memorycapacitor charging means inoperative and responsive to said secondtransition signal for unclamping;

means coupling said resistance signal and said minimum weld signal tothe high potential side of said unijunction transistor for firing sameto produce an end ratio signal upon concurrence of dropping of saidresistance signal to a preselected fraction of its peak value and thenext occurring negative going pulse in the minimum weld signal;

a switch responsive to discharge of said memory capacitor through saidunijunction for producing an output and a further one-shot multivibratorcoupled to the output thereof for providing an output pulse upon saiddischarge to said weld timer for terminating the weld interval beforetimin out of said weld timer. 2. The evice defined in claim 1 includinga quality indicator comprising a flip-flop circuit responsive to theoutput from said further one-shot multivibrator for assuming a secondconductive state and responsive to said first transition signal forassuming a first conductive state;

a semiconductor switch and an alarm for series energization thereby;

means normally energizing said semiconductor switch for actuating saidalarm and clamping means for disabling said energizing means so as toturn off said alarm, said clamping circuit being energizable by saidnegative buffer signal and by assumption of said second state by saidflip flop whereby said alarm will be energized when said negative buffersignal is terminated by timing out of the weld timer before saidflip-flop circuit is driven to said second state as a result of said endratio signal.

3. The device defined in claim 2 including a reset circuit for said flipflop comprising a manually actuable portion and an automatic portionresponsive to energization of said alarm switch and including timeddelay means and a pulse supply synchronized with the 0 point in thewelding current waveform for resetting said flip flop to its secondstate after a predetermined interval of alarm energization.

1. In a resistance welding machine including a generator for providing aresistance signal proportional to the instantaneous contact resistanceat the welding position and a weld timer for timing a preselected weldinterval and signaling the beginning and end thereof, apparatus forcausing the weld timer to terminate the weld interval before its normalconclusion in response to a drop of the weld resistance to a preselectedfraction of its peak value during the weld interval, indicatingcompletion of the satisfactory weld, such apparatus comprising thecombination: a source of energizing potential supplied at positive andnegative potentials related to a common potential; a buffer transistordisposed between said negative and positive potential lines forproviding a negative buffer signal upon initiation of weld time by saidweld timer; a one-shot multivibrator circuit responsive to said negativebuffer signal for providing a first transition signal during the initialportion of the weld interval and a second transition signal initiatedconcurrently with discontinuance of said first transition signal andterminating at the end of the weld interval, said first and secondtransition signals appearing on different output terminals of said oneshot multivibrator; a timing capacitor, means for charging saidcapacitor from said positive potential line and clamping meansconnecting said charging means to the output of said buffer transistorfor allowing said capacitor to charge prior to the weld interval and toclamp said charging circuit inoperative during the weld interval; a loadresistor across said timing capacitor for discharging same at apredetermined rate and buffer means providing the instantaneouspotential on said capacitor as an output thereof; a full wave rectifierenergized by an alternating current source synchronized with the weldcurrent in said welding machine and filter means for allowing thepotential across the full wave rectifier to drop essentially to zero atthe zero degree point of the welding current waveform but preventingsuch a reduction at the 180* point; a trigger circuit and means addingthe output of said full wave rectifier and said buffer means at theinput to said trigger circuit, said trigger providing a minimum weldsignal comprising a high potential maintained until said timingcapacitor discharges and thereafter interrupted by a negative goingpulse at each 0* point of the welding current waveform, the loadresistor of said capacitor being of sufficient size to cause saidcapacitor to time out upon completion of the minimum time period afterinitiation of the weld interval required to achieve a satisfactory weld;a unijunctioN transistor having a high potential side, a memorycapacitor coupled to the emitter thereof and means for supplying asignal proportional to the contact resistance at the welding head; meansfor applying a portion of said resistance signal to said memorycapacitor for charging same, means normally clamping said memorycapacitor charging means inoperative and responsive to said secondtransition signal for unclamping; means coupling said resistance signaland said minimum weld signal to the high potential side of saidunijunction transistor for firing same to produce an end ratio signalupon concurrence of dropping of said resistance signal to a preselectedfraction of its peak value and the next occurring negative going pulsein the minimum weld signal; a switch responsive to discharge of saidmemory capacitor through said unijunction for producing an output and afurther one-shot multivibrator coupled to the output thereof forproviding an output pulse upon said discharge to said weld timer forterminating the weld interval before timing out of said weld timer. 2.The device defined in claim 1 including a quality indicator comprising aflip-flop circuit responsive to the output from said further one-shotmultivibrator for assuming a second conductive state and responsive tosaid first transition signal for assuming a first conductive state; asemiconductor switch and an alarm for series energization thereby; meansnormally energizing said semiconductor switch for actuating said alarmand clamping means for disabling said energizing means so as to turn offsaid alarm, said clamping circuit being energizable by said negativebuffer signal and by assumption of said second state by said flip flopwhereby said alarm will be energized when said negative buffer signal isterminated by timing out of the weld timer before said flip-flop circuitis driven to said second state as a result of said end ratio signal. 3.The device defined in claim 2 including a reset circuit for said flipflop comprising a manually actuable portion and an automatic portionresponsive to energization of said alarm switch and including timeddelay means and a pulse supply synchronized with the 0* point in thewelding current waveform for resetting said flip flop to its secondstate after a predetermined interval of alarm energization.